Apparatuses that integrate printed circuit boards, such as payment terminals, comprise many securing devices and implement methods for ensuring that the apparatuses are used in compliance with the uses for which they are planned and that they comply with safety standards, currently called PCI (Payment Card Industry) PED (Pin Entry Device) standards, which are laid down by certification organizations.
For example, in the field of payment terminals for payment cards, manufacturers have developed solutions for protecting the electronic printed circuit board(s) (or PCB(s)), which may or may not be multi-layered, of payment terminals.
More specifically, a printed circuit board is a support, generally a plate or board, used to electrically connect a set of electronic components together in order to obtain a complex electronic circuit. This plate comprises an assembly of one or more thin layers of copper separated by an insulating material. The layers of copper are etched by a chemical method to obtain a set of tracks, terminated by lands. The printed circuit board (of prepreg+FR4) is often coated with a layer of colored varnish, which protects the tracks from oxidation and possible short-circuits. The tracks electrically connect different zones of the printed circuit board. The lands, once perforated, set up electrical links either between the components welded through the printed circuit board or between the different layers of copper (this is the technique known as the “via” technique). In certain cases, such non-perforated lands are used to weld surface-mounted components (SMCs).
Unfortunately, deterioration has been observed in electronic printed circuit boards. This deterioration takes the form of perforation, scraping or any other intrusive technique aimed at short-circuiting, cutting, snooping, etc. on a printed circuit board.
For example, a deterioration may correspond to the cutting of a track of the printed circuit board of a payment terminal in order to cancel the sending of information aimed at activating the “attack” mode of the secured processor of a payment terminal.
In order to protect the electronic printed circuit board, protective approaches have been developed. These protective approaches include known ways especially of using copper regions on printed circuit boards which, when connected together, act as switches. These copper regions may for example take the form of conductive mechanical elements used to link up the two constituent parts of a switch. These copper regions can also take the form of flexible printed circuit protecting the printed circuit board to be secured by defining a secured enclosure. In other words, the flexible printed circuit defines a protective volume or a “a quasi-hermetic cage” preventing the intrusion of any fraudulent person or intrusive tool.
Such flexible printed circuit boards comprise for example a sort of wire mesh triggered on electrical levels. The wire mesh comprises fine adjacent tracks covering the totality of the path to be protected. For example, one track may be connected to VCC and the other to GND, and a perforation of this wire mesh may lead to cut either of the links, even to a short-circuit between the two tracks, which would also be detected. Numerous variations of this device can be used. The detection can be made on voltage levels or on the compliance of signals travelling through these tracks.
The drawback of this type of approach based on the implementation of a secured enclosure comprising a flexible printed circuit board, or a conductive mechanical element, lies in the fact that, at the localized junction between the flexible circuit and the PCB, or the localized junction between the conductive mechanical element and the PCB, a space is left free and allows a fraudulent person if any to enter and damage or modify the correct functioning of the printed circuit board.
This problem is illustrated especially in FIG. 1 pertaining to the use of a secured enclosure comprising a flexible printed circuit board.
Referring to FIG. 1, a printed circuit board (10) called a PCB comprises electronic components (101, 102, etc). The printed circuit board 10 rests on a structure 20, generally made of plastic (also called an internal case as opposed to the external case of the terminal) which makes it possible to mount the components of the terminal (card reader, keyboard, screen, printer, etc).
As mentioned here above, to secure certain of these components, a flexible circuit (30) is directly welded to the surface of the PCB (10). As can be seen in the figure, this weld (301) makes it possible to both fix the flexible circuit (30) to the PCB (10) and also to set up a contact between the flexible circuit (30) and the input (111) and the output (112) of a switch of the PCB (10) so that the security processor can receive a signal coming from the flexible circuit. This contact zone is here below called a “switch zone” (110).
As shown schematically in FIG. 1, a free space (which is of course minimal but present) localized at the weld (301) nevertheless enables an attacker to introduce either a liquid or a thin object (320) or also a conductive foil between the PCB and the flexible circuit.
Such an introduction may for example be achieved by means of a preliminary lamination of the PCB. It is therefore possible, with this technique, to simulate the right contact localized at the “switch zone” between the flexible circuit and the PCB without the security processor present on the PCB being able to perceive it. In doing so, the switches of the “switch zone” are disconnected from the flexible circuit which is thereafter no longer capable of detecting any intrusion or deterioration.
Thus, as can be seen, although the use of a flexible circuit ensures a high level of securitization, especially against perforation, there remains certain weak points especially localized at the zone of contact between the PCB and the securing element (flexible printed circuit board or conductive mechanical element) which must be corrected in order to ensure the securing of the printed circuit board.
To date, the inventors have not identified any already existing simple solution with which to obtain efficient protection of an electronic printed circuit board against such localized intrusions at the “switch zone” where the contact between the PCB and the flexible printed circuit board is set up.